Satellite digital audio radio service (SDARS) is a relatively new satellite-based service that broadcasts audio entertainment to fixed and mobile receivers within the continental United States and potentially in various other parts of the world. Within an SDARS system, satellite-based transmissions provide the primary means of communication. However, certain urban settings may require a terrestrial transmission link to provide continuity of service, when satellite coverage is compromised due to the presence of obstructions. Reception of signals in the presence of objects, such as buildings, bridges, automobiles, etc., can distort a received signal even when the objects are not in the line-of-sight between a transmitter and a receiver. That is, signal rays reflecting off of various objects can sum constructively or destructively to produce interference patterns in the spectrum of a received signal. These distortion patterns, also known as multipath interference, can cause distortion in a demodulated signal.
In the case of digital communication systems, multipath interference may cause errors in a received data stream. While error correction coding can reduce the impact of multipath interference, critical portions of the received signal are often less robust and their preservation is required for successful recovery of the received signal. For example, a synchronization signal may require detection to allow data stream recovery. However, when multipath fading occurs and the synchronization signal is obscured, an earth-based receiver may not be able to synchronize with a satellite and/or a terrestrial ground station and, as such, the earth-based receiver cannot acquire a desired received signal.
In an effort to reduce the effect of multipath interference, at least some earth-based receivers have incorporated spatial diversity. That is, some earth-based receivers have incorporated separate satellite and terrestrial antennas, which optimize the reception of both the satellite and terrestrial components of the broadcast service. Due to the physical separation and difference in the antenna patterns and phase centers of the satellite and terrestrial antennas, spatially diverse versions of a desired received signal may be acquired. As such, if a desired received signal suffers impairment and its synchronization signal is unrecoverable, a different version of the signal from which the synchronization signal can be recovered may be obtained by use of a switching element that is internal to a given receiver. For example, the receiver may switch from the terrestrial antenna to the satellite antenna to receive a different version of the terrestrial signal, which is sufficiently unimpaired to allow signal recovery.
One SDARS system is expected to provide approximately one-hundred channels of music, news, sports, ethnic, children's and talk entertainment on a subscription-based service and may include other services, such as email and data delivery. In this system, the program material is transmitted from a ground station to two satellites that orbit over the continental United States in a geo-stationary orbit. This system also includes a number of terrestrial repeaters in large cities, where multipath interference is prevalent.
In various SDARS systems, a synchronization signal, contained within a received signal, is typically utilized by an earth-based receiver to determine the starting point for digital data framing. In systems that have a terrestrial bandwidth that includes multiple ensembles, each ensemble may include a narrowband synchronization signal that is susceptible to loss due to selective fading at the synchronism frequency. Due to the design of a typical SDARS earth-based receiver, once the synchronization signal is acquired, the receiver can sustain synchronization outages due to the fact that the receiver is capable of sustaining synchronization without the synchronization signal.
Thus, when a SDARS earth-based receiver initially receives a synchronization signal it attempts synchronization acquisition or lock. However, if an earth-based receiver cannot lock onto a synchronization signal, then it cannot receive such an SDARS broadcast. It should be appreciated that a mobile earth-based receiver may periodically experience nulls in the synchronization signal, due to the nature of multipath interference. These nulls, in general, do not interfere with the ability of the receiver to receive broadcast programming. After initial synchronism lock, frequent availability of the synchronism signal is largely unnecessary. However, should the mobile earth-based receiver stop in a null location or if a fixed earth-based receiver is located in a null location, then the earth-based receiver may not be able to initially acquire the synchronization signal from a start-up condition and, therefore, the receiver cannot obtain and provide such SDARS programmed content to a listener.
What is needed is an earth-based receiver that is capable of receiving programmed content even when the earth-based receiver is located such that its antenna experiences a null at a given frequency, which corresponds to the frequency of a synchronism signal.